What is the primary purpose of adding chromium to carbon steel?

The primary purpose of adding chromium to carbon steel is to enhance resistance to rust and corrosion. Chromium plays a vital role in forming a protective layer of chromium oxide on the surface of the steel. This layer acts as a barrier that protects the underlying metal from environmental factors, such as moisture and oxygen, which typically contribute to rust and corrosion.

In steel that contains at least 10.5% chromium, it is classified as stainless steel, which significantly improves its corrosion resistance compared to non-alloyed carbon steel. The incorporation of chromium not only prolongs the life of the steel in industrial and outdoor settings but also maintains its aesthetic appearance by preventing surface tarnishing or rusting over time. This property is particularly valuable in applications where durability and resistance to environmental degradation are crucial.

The other options relate to different properties of steel: while chromium can contribute somewhat to strength, especially in alloyed steels, its most defining attribute in this context is its ability to prevent corrosion. Malleability relates more to other alloying elements and production costs depend more on market conditions rather than a specific element's function in steel.